tests: Drain event loop in shell test

This make sure there's no lingering async/idle callbacks
of disposed objects.

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
10 jobs for resource-cleanups-part-2 in 34 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #346927
librem5
build:cross-debian-bullseye:i386

00:10:49

passed #346928
librem5
build:native-alpinelinux-edge

00:08:42

passed #346926
librem5
build:native-debian-bullseye

00:11:01

 
  Test+Docs
passed #346930
librem5
build-gtkdoc

00:03:58

passed #346932
librem5
check-license-headers

00:02:24

passed #346931
librem5
check-po

00:00:25

passed #346934
librem5
test:smoke:two-outputs

00:01:22

passed #346933
librem5
unit-test:native-alpinelinux-edge

00:00:45

passed #346929
librem5
unit-test:native-debian-bullseye

00:04:23

37.9%
 
  Package
passed #346935
librem5:arm64
package:deb-debian-bullseye:arm64

00:18:29